Heat 2 tablespoons olive oil in a cast iron skillet over medium heat. Add diced bell peppers and cook 5–7 minutes until tender but not mushy. Transfer to a bowl to cool slightly.
If using raw protein, cook it in the same skillet with 1 tablespoon olive oil until done; for rotisserie chicken, shred and set aside. Season as desired (optional).
Set up an assembly station and lay out all 8 tortillas on a clean surface.
On half of each tortilla, add about 1/4 cup cheese, 3 ounces cooked protein, about 3 tablespoons peppers, 3 tablespoons black beans, then another 1/4 cup cheese. Fold to form a half-moon.
Warm the skillet over medium heat. Lightly oil if needed. Cook each quesadilla 3–4 minutes per side until golden and the cheese is melted.
Transfer cooked quesadillas to a wire cooling rack and cool at least 10 minutes to let steam escape and keep them crisp.
Cut in halves if desired and portion into glass meal prep containers with dividers. Keep salsa in a separate compartment or small side container.
Label each container with the prep date and contents.
Refrigerate for up to 5 days, or flash-freeze on a baking sheet 30 minutes and transfer to freezer bags for up to 2 months.
To reheat: skillet method (best texture) 1–2 minutes per side over medium heat; microwave 60–90 seconds; from frozen bake at 375°F for 12–15 minutes or microwave 4–5 minutes.